1. Field of the Invention
The present invention relates to a duty-to-voltage amplifier, a FM receiver and a method for amplifying a peak of a MPX signal; more specifically, relates to a duty-to-voltage amplifier, a FM receiver and a method for amplifying a peak of a MPX signal via a current splitter.
2. Descriptions of the Related Art
Frequency modulation (FM) is frequently used for the purpose of wireless communications. To receive FM signals successfully, a particular receiver is required. As shown in FIG. 1, a FM receiver 1 of the prior art comprises an intermediate frequency (IF) filter 101, a FM demodulator 103 and a stereo decoder 105. The IF filter 101 filters a FM signal 100 which is derived from a radio frequency filter (not shown) filtering an RF signal, and generates a FM modulated signal 102 and a relative signal strength indicator (RSSI) 104. The FM demodulator 103 generates a MPX signal 106 by demodulating the FM modulated signal 102. The MPX signal 106 comprises a pilot signal and an audio signal which is either stereo or mono. The stereo decoder 105 retrieves the audio signal 108 by decoding the MPX signal 106 in an appropriate decoding process according to the pilot signal. The audio signal 108 is then transmitted to a loudspeaker to play sound.
The RSSI 104 is used to indicate the strength of the RF signal. When RSSI 104 indicates that the RF signal is too weak to be processed, the FM receiver 1 trigger a soft-mute operation in which the MPX signal 106 is adjusted smaller and smaller until sound disappears. The stereo decoder 105 comprises an amplifying circuit 2, as FIG. 2 shows, that may perform the soft-mute operation in response to the RSSI 104. The amplifying circuit 2 comprises two resistors R1 and R2, a transistor T1, an operational amplifier 201, and a compared amplifier 203. The compared amplifier 203 compares a reference signal 200 and the RSSI 104 to generate a compared signal 202. The compared signal 202 is inputted to the gate of the transistor T1 so the resistance RT between the source and the drain may be controlled by the compared signal 202. The transform equation, i.e. the gain, of the amplifying circuit 2 is shown as followed:
            Vout      Vin        =          -                                    R            ⁢                                                  ⁢            2                    //          RT                          R          ⁢                                          ⁢          1                          RT    =          1                        μ          n                ⁢                  C          ox                ⁢                  W          L                ⁢                  (                                    RSSI                                                                                ⁢                ′                                      -            Vout            -            Vt                    )                    
wherein
      μ    n    ⁢      C    ox    ⁢      W    L  denotes a physical parameter of the transistor T1, Vt denotes a threshold voltage of the transistor T1, and RSSI′ denotes the voltage of the compared signal 202. When a soft-mute operation is triggered, the resistance RT is much smaller than R2. The transform equation may therefore be re-written as:
      Vout    Vin    ≈            -              1                  R          ⁢                                          ⁢          1                      ·          1                        μ          n                ⁢                  C          ox                ⁢                  W          L                ⁢                  (                                    RSSI                                                                                ⁢                ′                                      -            Vout            -            Vt                    )                    
Since the MPX signal 106 is inputted to the Vin end, the peak of the MPX signal 106, outputted from the Vout end, can be reduced in response to the voltage of the compared signal 202.
However, such arrangement causes that the gain is influenced by the peak of the MPX signal 106, and hence makes distortion serious. Accordingly, a solution to solve the problem of distortion when a soft-mute operation is triggered is desired in the industrial field.